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The metonic calendar incorporates knowledge that 19 solar years and 235 lunar months are very near equal, thus lunar periods often, but not unconditionally, repeat on the same day of the year as 19 years previous. This system was based on calculations made by Meton using his own observations of the summer solstice in 432 BC, and an observation made by Aristarchus 152 years later.[1] Meton's observations were made in collaboration with Euctemon about whom nothing else is known. The Greek astronomer Callippus continued the work of Meton, proposing what is now termed the Callippic cycle. The Callippic cycle is 76 years long, four Metonic cycles. Callippus refined the Metonic cycle, deducting one day every four Metonic after 940 synodic lunar periods to synchronize the lunar calendar with the year.

The world's oldest known astronomical calculator, the Antikythera Mechanism (2nd century BC), performs calculations based on both the Metonic and Callipic calendar cycles, with separate dials for each.[2][3]

The foundations of Meton's observatory in Athens are still visible just behind the podium of the Pnyx, the ancient parliament. Meton found the dates of equinoxes and solstices by observing sunrise from his observatory. From that point of observation, during the summer solstice, sunrise was in line with the local hill of Mount Lycabetus, while six months later, during the winter solstice, sunrise occurs over the high brow of Mount Hymettos in the southeast. So from Meton's observatory the sun appears to move along a 60° arc between these two points on the horizon every six months. The bisectrice of the observatory's solstitial arc lies in line with the Acropolis. These topological features are important because the summer solstice was the point in time from which the Athenians measured the start of their calendar years. The first month of the new year, Hekatombaion, began with the first new moon after the summer solstice.[4]

What little is known of Meton come through ancient historians. According to Ptolemy, a stela or table erected in Athens contained a record of Meton's observations, and a description of the Metonic cycle.[1] None of Meton's works survive.